Hinsberg test

The Hinsberg test is a classic wet-chemistry way to tell whether an unknown amine is primary, secondary, or tertiary — back before spectroscopy, this was how you sorted them out with nothing but a test tube and your eyes. The reagent is benzenesulfonyl chloride (C6H5-SO2-Cl), shaken with the amine in aqueous base, and the answer is read from what dissolves and what does not.

It works because the three amine classes behave differently toward the sulfonyl chloride. A primary amine (one N-H to spare after reacting) forms a sulfonamide whose remaining N-H is acidic; base deprotonates it, so the product dissolves as a clear solution — and adding acid later precipitates it back. A secondary amine forms a sulfonamide too, but it has no N-H left on that nitrogen, so it cannot be deprotonated and shows up as an insoluble precipitate even in base. A tertiary amine has no N-H to begin with and cannot form a stable sulfonamide at all, so (classically) it stays unreacted — appearing as an insoluble oil or layer that dissolves only when acid is added (protonating the basic amine).

So in plain reading: clear solution that precipitates on acidifying means primary; precipitate that stays insoluble in base means secondary; an oily layer unreacted in base but dissolving in acid means tertiary. The honest caveats: the tertiary-amine result is the least clean part of the test and can mislead (tertiary amines can react with the sulfonyl chloride in unexpected ways), and modern labs simply run an NMR or IR spectrum instead. Still, the test beautifully ties together sulfonamide formation, N-H acidity, and amine basicity in one tube.